1. Field of the Invention
The invention relates to apparatus for performing optical emission spectroscopy and, more particularly, to apparatus for performing optical emission spectroscopy to analyze plasma enhanced processes. Within a semiconductor wafer processing system
2. Description of the Background Art
Optical emission spectroscopy has been used in the prior art for monitoring and analyzing the characteristics of a plasma within a dry etch reaction chamber of a semiconductor wafer processing system. Optical emission spectroscopy (OES) is accomplished using a monochromator that is coupled to a transparent viewing window of the reaction chamber. Light generated by the plasma is carried by an optical fiber to the monochromator, and the monochromator selects a particular wavelength for analysis using a diffraction grating. The particular wavelength is disbursed from the grating at a specific angle to a photomultiplier detector or some other form of light detector. The photomultiplier detector (PMD) or other form of photo detector produces an electrical voltage representing the magnitude of energy at the particular wavelength selected by the monochromator. This voltage is typically analyzed by a computer system to detect and/or control the end point of a plasma enhanced etch process. Such OES systems have many uses in analyzing, characterizing and otherwise monitoring a plasma within a reaction chamber of a semiconductor processing system. Such OES systems are disclosed in U.S. Pat. Nos. 5,288,367 issued Feb. 22, 1994; 5,308,414 issued May 3, 1994; and 4,859,277 issued Aug. 22, 1989.
Typically, the monochromator selects a wavelength from the broad spectrum optical signal produced by the plasma using a diffraction grating. Such a defraction grating is inefficient in its selection process and significantly degrades the magnitude of the selected wavelength. Consequently, the monochromator of this sort is inefficient and the number of photons that arrive on the photo sensitive area of the detector is significantly less than the number actually available for detection, resulting in high shot noise content within the detected signal.
Therefore, there is a need in the art for a more efficient optical signal detector that may efficiently select and analyze a band of wavelengths from the broadband optical signal generated by a plasma within a reaction chamber of a plasma enhanced semiconductor wafer processing system.